1. Field of the Invention
The present invention relates generally to transmission power control of a downlink shared channel (DSCH) in an asynchronous mobile communication system, and in particular, to an apparatus and method for controlling the transmission power of a DSCH for a user equipment (UE) in a handoff region.
2. Description of the Related Art
The DSCH used in the European asynchronous future mobile communication system known as “WCDMA” is shared by multiple UEs. The DSCH is designed to transmit packet data or other high rate data to UEs in 10-ms radio frames. The DSCH supports a variable data rate for data transmitted at a frame level and can be power-controlled on a slot basis like the dedicated channel (DCH) established between a node B and a UE in WCDMA. A radio frame is a signal transmission unit of 10 ms for WCDMA and includes 15 slots. The DSCH transmits user data only and its power control is carried out by a downlink dedicated channel (DL_DCH) assigned together with the DSCH. The DSCH can be transmitted to a single UE in several frames or in a single frame. Which UE to transmit the DSCH to and when to transmit the DSCH to the UE are scheduled by the higher layer.
For thorough understanding of the DSCH, its structure will be described with reference to FIG. 1A. In FIG. 1A, reference numeral 101 denotes a 10-ms DSCH radio frame and reference numeral 103 denotes a slot. The DSCH radio frame 101 includes 15 slots and the single DSCH slot 103 is 2560 chips long. The volume of information transmitted in the DSCH slot 103 is inversely proportional to the spreading factor (SF) applied to the DSCH slot 103, ranging from 4 to 256.
FIG. 1B illustrates the structure of a DL_DCH assigned to a UE together with the DSCH shown in FIG. 1A by a node B. In FIG. 1B, reference numeral 111 denotes a DL_DCH radio frame. One DL_DCH slot includes Data 1 113, TPC (Transmit Power Control) 112, TFCI (Transmit Format Combination Indicator) 114, Data 2 115, and Pilot 116. The DL_DCH slot may have a different structure according to the lengths of Data 1, TPC, TFCI, Data 2, and Pilot.
Data 1 113 and Data 2 115 are called a downlink dedicated physical data channel (DL_DPDCH) for transmitting user data and signaling information from the higher layer. TPC 112, TFCI 114, and Pilot 116 collectively form a downlink dedicated physical control channel (DL_DPCCH). TPC 112 transmits a command for controlling the transmission power of an uplink channel directed from the UE to the node B, TFCI 114 transmits a codeword notifying transmission of transport channels at different data rates on the DL_DCH when the case occurs, and Pilot 116 allows the UE to measure the transmission power of a downlink channel for power control of the downlink channel. Transport channels in the description of TFCI 114 function to connect the higher layer and the physical layer that physically controls the data transmission.
For power control of the DSCH in WCDMA, the UE receiving the DL_DCH 111 measures Pilot 116 shown in FIG. 1B and transmits a TPC command to the node B. The UE determines whether the reception power is acceptable from the pilot measurement. According to the power control command from the UE, the node B controls the transmission power of the DL_DCH and sets the transmission power of the DSCH to an appropriate level considering the data rates of the DL_DCH and the DSCH. The difference in transmission power between the DL_DCH and the DSCH is dependent on the data rates of the channels and can be easily calculated by known procedures.
Transmission of a TPC on a slot by slot basis from the UE enables power control for the DL_DCH on a slot by slot basis. This implies that the transmission power of the DSCH can also be controlled on a slot basis.
FIG. 2 illustrates the flow of uplink and downlink signals in the case where the UE receiving the DSCH is located in a soft handoff (SHO) region. For simplicity of description, a system with, only two node Bs are considered here. The SHO occurs when the UE moves to a region where it receives a signal from both a source node B and a source target node B. In the SHO region, the UE communicates with both the node source and target B for a predetermined time. As the UE leaves the service region of the source node B, the quality of a signal from the source node B reaches an unacceptable level. The UE then releases channels from the source node B offering bad signal quality, connecting a call to the target node B offering good signal quality. This is called an SHO.
When the UE reaches the SHO region, it sums the transmission power of the source node B and the target node B and sets their transmission power to a mean value to allow the decrease of the transmission power of the node Bs to hand off a call without interruption. As a result, the transmission power of node Bs broadcasting signals to UEs within its coverage area is decreased, thereby reducing the influence of interference on adjacent UEs and node Bs.
This SHO procedure will be described in more detail referring to FIG. 2. Node B1 201 transmits a DSCH and a corresponding DL_DCH to a UE 211 and node B2 203 comes to transmit a DL_DCH to the UE 211 as the UE 211 moves toward it. A set of node Bs capable of transmitting signals to the UE 211 is called an active set. As the UE 211 receiving the DSCH enters the SHO region, the following problems may occur.
While the UE 211 receives both the DSCH and the DL_DCH from node B 1 201, it receives only the DL_DCH from node B2 203. The DSCH does not support the SHO because (1) it transmits high rate data relative to the DL_DCH, occupying more channel resources, (2) all node Bs in the active set must be provided with an SCH-supporting algorithm to support the SHO of the DSCH, which requires synchronization between the node Bs, (3) the asynchronous operations of node Bs in the asynchronous mobile communication system may cause timing-related problems, and (4) accurate scheduling of using time points for UEs due to the nature of the DSCH makes it difficult for a different node B to transmit the DSCH to the UE.
The DL_DCHs received from node B1 201 and node B2 203 are soft-combined for interpretation in the UE 211. The soft combination is a process of combining signals received at the UE 211. The purpose of soft combining is to reduce the influence of noise on received signals by summing the same information from different paths prior to interpretation. The soft combining operation is feasible only when the UE 211 receives the same information from different node Bs. If different information is received from each node B, soft combining merely increases noise components. Except for TPCs 112, the DL_DCHs are soft-combined. As the UE 211 roams, the signal strength of node B1 201 at the UE 211 may be strong, while that of node B2 203 may be weak, or vice versa. Due to the resulting possible difference between the TPCs, the TPCs of the DL_DCHs are interpreted separately without soft combining.
In determining a TPC for an uplink dedicated channel (UL_DCH) as shown in FIG. 2, the UE 211 sums signals received from node B1 201 and node B2 203 and checks whether the received signal level is acceptable. When the transmission power of the DSCH directed to the UE 211 in the SHO region is determined, problems as described below may arise from the TPC determination based on the mere sum or weighted sum of the received signals.
In the case where the UE 211 is located out of the SHO region and thus communicating with only the source node, the transmission power of the DSCH for the UE 211 is determined by adding the transmission power of the DL_DCH to a value reflecting the difference between the data rates between the DSCH and the DL_DCH. That is, the DSCH transmission power is bound with the DL_DCH. As the DL_DCH transmission power increases, the DSCH transmission power also increases, and vice versa. The DSCH can be transmitted to the UE 211 adaptively to the channel environment between the source node B1 201 and the UE 211. However, if the UE 211 is located in the SHO region, signals from other node Bs in the active set as well as a signal from the source node B1 201 transmitting the DSCH are involved in determining the TPC for the UL_DCH. To clarify the point with reference to FIG. 2, the DSCH transmission power must be determined considering the channel conditions between the UE 211 and node B1 201 and the DL_DCH transmission power must be determined considering the channel conditions between the UE 211 and node B1 201 and the UE 211 and node B2 203. In the prior art, since the DSCH transmission power is determined by adding a predetermined power value to the DL_DCH transmission power, application of a TPC according to the additional channel condition between the UE 211 and node B2 203 results in transmission of the DSCH with power above or below a desired power level. Another problem is encountered when node B1 201 transmits the DL_DCH with lower transmission power to the UE 211 in the SHO region than needed for transmitting the DL_DCH by itself. In this case, node B1 201 cannot apply the difference in transmission power between the DSCH and the DL_DCH for the non-SHO regions.
Many techniques have been proposed to solve the DSCH transmission power-related problems in the SHO region. One of the proposed techniques, shown in FIG. 3, utilizes site selection diversity transmit (SSDT) according to the WCDMA standards for DSCH transmission power control. For better understanding, it is supposed that an active set s includes two node Bs.
In the SSDT scheme, a temporary identification (ID) is assigned to each node B in the active set of a UE 311 located in an SHO region and a node B that can offer the best signal quality to the UE 311 is selected. Only the selected node B transmits a DL_DPDCH to the UE 311 and the other node B transmits only a DL_DPCCH to the UE 311, thereby reducing interference caused by simultaneous reception of DL_DPDCHs at the UE 311 from all node Bs in the active set to support the SHO. The node B transmitting a DL_DPDCH is called a primary node B, which is periodically updated, based on the measurement information in the UE 311. The primary node B is updated by transmitting its temporary ID to the other node Bs in the active set.
To control the DSCH transmission power using SSDT, the UE 311 receives common pilot channels (CPICHs) from node B1 301 and node B2 303 and determines a primary node B by comparing the pilot signal strengths of the CPICHs. Then the UE 311 transmits the temporary ID of the primary node B to each node B. A node B transmitting a DSCH among the node Bs receiving the temporary ID receives the temporary ID several times for a predetermined period and checks how many times the temporary ID indicates the node B. The node B determines whether to transmit the DSCH in a primary node B mode or in a non-primary node B mode.
For example, node B1 301 transmits both a DL_DCH and a DSCH to the UE 311. Node B2 303 is newly included in the active set of the UE 311 and transmits only a DL_DCH to the UE 311. After comparing the signal strengths of CPICHs from the node Bs, the UE 311 transmits the temporary ID of a primary node to node B1 301, as the primary node and node B2 303. If the temporary ID designates node B1 301, node B1 301 determines the transmission power of the DSCH considering the TPC of a UL_DCH and factors caused by the movement of the UE 311 to the SHO region, for example, a power offset reflecting the transmission power decrement of the DL_DCH. That is, it is determined whether the DSCH transmission power is to be increased or decreased based on a TPC received from the UE 311. In the case where node B1 301 is a primary node B, the DSCH power control is performed in the same manner as for the UE 311 in a non-SHO region except that a required power offset is applied due to factors such as the transmission power decrement of the DL_DCH.
On the other hand, if node B2 303 is selected as a primary node B, node B1 301 transmits the DSCH with a fixed power offset applied to the UE 311, judging that the UE 311 becomes remote or the channel condition is bad. That is, node B1 301 transmits the DSCH with the preset power offset applied to the UE 311, neglecting a TPC received from the UE 311.
The above-described DSCH transmission power control relying on SSDT has the shortcomings described below. (1) When the UE 311 enters the SHO region, the transmission power of an individual DL_DCH from each node B is less than that of a DL_DCH transmitted by only one node B and the difference varies according to the number of node Bs in the active set. Moreover, since a TPC transmitted from the UE 311 for downlink power control is determined after the DL_DCHs from the node Bs are combined and then it is determined whether the signal quality is acceptable or not, the TPC determination is influenced by the channel condition between the UE 311 and the other node Bs as well as the channel condition between the UE 311 and the node B transmitting the DSCH. Therefore, although the node B transmitting the DSCH is a primary node B, there may exist an error between the DSCH transmission power determined based on the TPC of a UL_DCH and a desired DSCH transmission power. (2) When the UE is located in the SHO region, the DSCH-transmitting node B transmits the DSCH with a different fixed power offset according to whether it is a primary node B or not. If the DSCH-transmitting node B is not designated as a primary node B while the balance of reception power is set among node Bs in the active set, the DSCH may be transmitted with overpower. If the DSCH-transmitting node B becomes a primary node B, the DSCH can be transmitted with underpower. The application of a different fixed power offset according to a primary node B or a non-primary node B may bring about an error between the real DSCH transmission power and the desired DSCH transmission power.